Tensile Fabric Structures Design, Analysis, and Construction PREPARED BY Task Committee on Tensioned Fabric Structures EDITED BY Craig G. Huntington SPONSORED BY Structural Engineering Institute of ASCE Published by the American Society of Civil Engineers Cataloging-in-Publication Data on file with the Library of Congress. American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia, 20191-4400 www.pubs.asce.org Any statements expressed in these materials are those of the individual authors and do not necessarily represent the views of ASCE, which takes no responsibility for any statement made herein. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. 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Front Cover Stanford University Aquatic Center Architect: ELS Architecture & Urban Design Fabric Canopy Design & Engineering: Huntington Design Associates Back Cover Hampton Roads Convention Center Porte Cochere Architect: HOK Fabric Structure Design & Engineering: FTL Design Engineering Studio Preface Tensile membrane structures are part of a unique technology that gives designers, architects and engineers the ability to experiment with form and create exciting solutions to conventional design problems. These structures are not only visually exciting, but are environmentally sensitive and economically competitive as well. Membrane translucency provides more light than most indoor activities require and creates an attractive "glow at night.” The state-of-the art materials, typically PTFE- coated fiberglass and vinyl-coated polyesters, are inherently waterproof and require very little maintenance. Because the materials are lightweight, these structures are extremely efficient in long span applications and are often constructed with substantial savings in the foundation and supporting structure costs. As an added bonus, they do more than just transmit forces to the ground. They serve as the primary architectural form determinant and provide much of the building envelope. Conventional structures rely on internal rigidity (stiffness) to achieve stability and to carry loads. Fabric structures constructed of elements that have little or no bending or shear stiffness (cables and membranes) must rely on their form and internal tensile forces to carry loads. What makes these structures more complicated to design than their conventional counterparts is that they tend to be highly non-linear in their behavior and their shape is not known when the design begins. The non-linearity is a result of significant changes in geometry that usually occur under load, even though the materials remain, more or less, linearly elastic after the initial set. This change in geometry is a desirable quality, since if properly designed, tensioned fabric structures will increase their capacity to carry load as they deform. In fact, these structures are capable of maintaining a very high ratio of applied loads to self-weight, in contrast to steel or concrete structures of the same spans. In the last 20 years, great advances have been made in this field. Today we have very sophisticated software for the analysis and design of membrane structures. Not too long ago most computer programs involved with these structures were generated by companies that were very reluctant to share or sell them. At the time of this writing, software that varies in price, capability, and ease of use is available from more than a dozen sources. This publication describes the materials, design, and behavior of tensioned fabric structures. Chapter 1 reviews the history of the technology. Chapter 2 describes the overall design and construction process and discusses the role of each participant in the project from inception through completion. Chapter 3 treats the properties of various fabrics and films. In Chapter 4 types of loads and their effects are discussed. The design of a tensile membrane structure can be separated into two distinct phases: shape determination (sometimes called form-finding) and analysis under different loads. Shape determination requires the "design" of a structure whose form is not known in advance; changes in internal pre-stress will change the shape of the overall v vi TENSILE FABRIC STRUCTURES structure. Analysis of the system requires the solution of equations for the deformed configuration, a shape that is also unknown in advance. If the stresses in the elements are too high or if the deformations are greater than acceptable, the designer is free to change the shape of the structure by revising the pre-stress or by modifying the boundary conditions. These subjects are discussed in Chapters 5 and 6. Once the structure is designed, the final steps to its completion are fabrication and erection. Chapter 7 describes the connections between the various materials and the attachments to the supporting structure. In Chapter 8, the non-structural issues unique to tensile membrane structures are presented. Patterning, the process of selecting an arrangement of two-dimensional panels to develop the three-dimensional surface, is discussed in Chapter 9. Finally, the erection of the structure, which requires careful handling of the materials as well as knowledge of the behavior of the structure, is also treated in Chapter 9. In conclusion, it should be emphasized that this document does not purport to be a comprehensive treatment of the subject. Instead, the purpose of this publication is to assist design professionals and builders in understanding the basic design principles, materials, fabrication methods, and erection procedures utilized with these structures. Experienced designers are presented with ideas that may help them further understand their craft and hone their skills. Novices are offered a wide range of introductory information to assist them in entering the exciting field of tensile membrane structures. Perhaps with both the expert and the beginner, we can inspire the creation of more of these wonderful structures. The Authors are members of the Tensioned Fabric Structures Task Committee of the Special Structures Committee of the Committee on Metals, 2010. Contents Preface ............................................................................................................................v Acknowledgements ..................................................................................................... vii Chapter 1 History and Development of Fabric Structures .....................................1 Chapter 2 The Design and Construction Process ...................................................25 Chapter 3 The Material Characteristics of Fabrics ...............................................32 Chapter 4 Loads ........................................................................................................51 Chapter 5 Form Determination ...............................................................................56 Chapter 6 Load Analysis ..........................................................................................94 Chapter 7 Connections ...........................................................................................104 Chapter 8 Non-Structural Performance Considerations ....................................126 Chapter 9 Fabrication and Construction .............................................................138 Appendix 1 Glossary of Terms ...............................................................................151 Appendix 2 Wind-Tunnel Studies of Tensioned Membrane Structures .............159 Bibliography .............................................................................................................165 Index ..................................................................................................................................... 173 Chapter 1 History and Development of Fabric Structures 1.1 Traditional Tent Forms The tent has been the dwelling in one form or another for most nomadic peoples from the Ice Age to the present. Vegetation permitting, the most common supports for tents were tree branches or the trunks of saplings. The heavier of these were sometimes left behind because of transportation problems. The skin or velum of early tents used animal hides or, less frequently, birch bark pieces or latticed leaf fronds. Later, these were replaced by woven materials such as wool or canvas. Contemporary materials include aluminum, fiberglass, and steel for the supporting elements and highly sophisticated synthetic fabrics for the velum. Figure 1-1 Kibitka (Sketch by the author) Until quite recently most tents consisted of three basic forms: the conical or tepee shape, the widespread kibitka or yurt that has cylindrical walls and a conical or domical roof as shown in Figure 1-1, and the "black" tent that has the velum tensioned into saddle shapes as shown in Figure 1-2. The black tent gets its name from the goat hair used to weave the velum. (The gable-roofed, ridge-type tent saw little use in ancient times but became a popular and durable military form beginning in the 18th century. It could be considered as an adaptation of the kibitka form to a rectangular plan.) Of the three basic forms, the conical tepee form is the oldest and saw widespread use across northern Europe, northern Asia, and North America. The conical kibitka shape was prevalent as far back as 2000 B.C., and even now it is used more than any other dwelling form in the world. The same shape executed in vines and straw is found throughout Africa and South America. This tent form developed in a wide band from the eastern Mediterranean region to Mongolia. Its shape has been the one most copied or adapted for later tents. For example, a parasol roof shape derived from the 1 32 2 TENSILE FABRIC STRUCTURES kibitka was used in the military tents of eastern European countries in the 18th Century and before. Figure 1-2 “Black” Tent (Sketch by the author) The "black” tent is probably about as old as the kibitka form and, like it, is still much used today. The loosely woven cloth permits the passage of air yet provides a high degree of shade, appropriate for its use in hot arid climates. It developed in Asia from Iran and Afghanistan and later spread to northern Africa. One can easily contrast the black tent of the warmer arid regions and the tepee shape of the northern climates. The steeply sloped sides of the latter form do not easily collect snow and provide a natural chimney for the necessary fire within. The low profile and shallow slopes of the black tent make it resistant to the desert winds. Of the three basic shapes, the black tent is the only one in which the form is not completely determined by its supporting framework. In the first two, the velum serves only as a barrier to the elements and is not an integral part of the structural system. In the black tent, however, the amount of tension (or prestress) in the velum establishes its scalloped form and provides stability for the supporting elements. In this manner and because of its basic anticlastic surface, it is highly related, from a structural standpoint, to contemporary tensioned fabric structures. Another structurally interesting tent form is the "envalet," popular in Spain for several decades after 1900. These tents had a clear span of about 30 meters and were erected annually for village festivals. Tall wood poles were placed around the perimeter of the roof and ropes were suspended across the rectangular plan so that the fabric could be suspended from above (Figure 1-3). TENSILE FABRIC STRUCTURES 3 Figure 1-3 “Envalet” Tent (Sketch by the author) One of the largest tents ever constructed was the one used in 1925 for the National Congress of India led by Mahatma Gandhi. It provided shade for thousands of delegates and visitors. Wooden poles were used to support the hand-woven membrane. The largest wall tents were the traveling circus "big tops" popular in the U.S. from the early 19th Century. Harnessed elephants were often used to pull the supporting poles into place as the tents were set up and taken down many times in the course of a single season. In the 1950s, these reached their maximum size, covering more than one hectare. Shortly thereafter, circuses abandoned the tents as more cities were able to provide a rigid-roofed civic center or coliseum. 1.2 Air Structures The air-supported roof provides an economical way to achieve long spans. Such structures were first proposed in 1917 by William Lanchester of England for use as field hospitals. He received a patent but never constructed one. In 1946 Walter Bird pioneered the radome; the first one was constructed of neoprene-coated fiberglass with a diameter of 15 meters (Figure 1-4). By the 1960s his Birdair Company was building them with spans of more than 60 meters using a laminated DACRON fabric with a HYPALON coating. In 1958 Walter Bird constructed the McBac Arts Center Theatre in Boston. Designed by architect Carl Koch and engineered by Weidlinger Associates, it was intended to be erected each summer. The roof consisted of an air-inflated, lens-shaped "pillow" supported by a steel compression ring. (The Birdair Company later grew to construct 4 TENSILE FABRIC STRUCTURES most of the large fabric structures in the United States in the latter half of the 20th Century.) Figure 1-4 Walter Bird on top of the first radome near Buffalo, NY in 1946 (Photograph with permission from Milt Punnett) The 1970 World's Fair site in Osaka, Japan provided the impetus for rapid developments in fabric structures. The poor soil conditions and the threat of seismic shaking both suggested the use of lightweight structures. From a structural standpoint, the most significant building at the Fair was the U.S. Pavilion designed by the architecture firm of Davis and Brody and engineer David Geiger of the Geiger- Berger firm (Figure 1-5). The low-profile, cable-restrained, air-supported structure was made of vinyl-coated fiberglass spanning to an oval-shaped concrete compression ring. It provided 10 800 square meters of column-free exhibit space. By using a super-ellipse for the ring and a diagonal cable pattern, Geiger was able to greatly reduce the bending forces in the ring. This simple, innovative structure was actually the result of major budget cutbacks that had sacked two previous designs by the architects. TENSILE FABRIC STRUCTURES 5 Figure 1-5 U.S. Pavilion, Osaka World Fair, Japan, 1970 (Photograph courtesy of Geiger Engineers) About this time, Harold Gores of the Educational Facilities Laboratory (EFL), an arm of the Ford Foundation, was looking for ways to provide temporary college athletic facilities to accommodate the arriving baby boomers. The search was on for a fabric for use in air-supported roofs that was very strong but resistant to both fire and ultraviolet deterioration. A team of John Effenberger (DuPont), Malcolm Crowder (Owens-Corning), John Cook (Chemical Fabrics Corp.), and David Geiger proposed using fiberglass coated with polytetrafluoroethylene (PTFE), better known as TEFLON, a product developed by DuPont. (All of the large, air-supported roofs and almost all of the larger tensioned fabric roofs have been constructed of this material.) This material was originally developed by NASA for space suits. Hal Gores convinced the presidents of two private colleges to gamble on the new structural system. The Steve Lacy Field House at Milligan College in Tennessee was constructed in 1972-75. It was a cable-restrained, insulated roof with a diameter of 65 meters. The Thomas H. Leavey Activities Center at Santa Clara College in California was completed in 1973 and consisted of two oval-shaped structures, the larger being 91 x 59 meters in plan. From the outset, the Milligan roof encountered inflation difficulties and was replaced by a rigid steel frame in 1986. In 1975 the Silverdome at Pontiac, Michigan was completed, measuring 220 x 159 meters and providing a clear span exceeding those of the Astrodome and the Superdome in Houston and New Orleans, respectively. Smaller college-facility domes were constructed in the next few years: UNIDome at the University of Northern Iowa (1976), Dakota Dome at the University of South Dakota (1979), O'Connell Center at the University of Florida, and the Sundome at the University of South Florida (both 1980). Following Pontiac, five more of the large domes were built: Carrier Dome at Syracuse University (1980); Metrodome in Minneapolis (1982); B. C. Place, Vancouver, Canada (1983); Hoosier Dome in Indianapolis; and Tokyo Dome (1988). Almost all of these were engineered by David Geiger and built